1. Technical Field
The present invention relates to a liquid crystal device, a driving circuit of a liquid crystal device, a method of driving a liquid crystal device, and an electronic apparatus.
2. Related Art
Liquid crystal devices are known examples of display devices. For example, such a liquid crystal device may include a liquid crystal panel and a backlight that supplies light to the liquid crystal panel.
A liquid crystal panel is configured to include an element substrate, a counter substrate disposed opposite the element substrate, and liquid crystal provided between the element substrate and the counter substrate.
The element substrate includes a plurality of scanning lines and a plurality of capacitive lines provided alternately with predetermined gaps therebetween, a plurality of data lines that cross the plurality of scanning lines and the plurality of capacitive lines and are provided with predetermined gaps therebetween, a scanning line driving circuit connected to the plurality of scanning lines, a data line driving circuit connected to the plurality of data lines, and a capacitive line driving circuit connected to the plurality of capacitive lines.
Pixels are provided at intersections of the scanning lines and the data lines. A pixel is configured to include a pixel capacitor having a pixel electrode and a common electrode, a thin film transistor (hereinafter, referred to as ‘TFT’) serving as a switching element, and a storage capacitor having one electrode connected to a capacitive line another electrode connected to a pixel electrode. A plurality of pixels are arranged in a matrix to form a display area. In each of the pixels, the scanning line is connected to a gate of the TFT, the data line is connected to a source of the TFT, and the pixel electrode and the other electrode of the storage capacitor are connected to a drain of the TFT.
The capacitive line driving circuit supplies a predetermined voltage to each capacitive line.
The scanning line driving circuit supplies a selection voltage for selecting scanning lines to each of the scanning lines in a predetermined order. When a selection voltage is supplied to a scanning line, a TFT connected to the scanning line is turned on.
The data line driving circuit supplies an image signal to each of the data lines and writes an image voltage based on the image signal into a pixel electrode through the TFT that is in the ON state. In this case, the data line driving circuit alternately performs positive-polarity writing and negative-polarity writing. Specifically, the data line driving circuit supplies an image signal, which has a higher voltage (hereinafter, referred to as ‘positive polarity’) than a voltage of a common electrode, to a data line so as to write an image voltage based on the positive-polarity image signal into a pixel electrode (positive-polarity writing) and supplies an image signal, which has a lower voltage (hereinafter, referred to as ‘negative polarity’) than the voltage of the common electrode, to a data line so as to write an image voltage based on the negative-polarity image signal into a pixel electrode (negative-polarity writing) in an alternate manner.
The counter substrate has R (red), G (green), and B (blue) color filters provided for each pixel.
The liquid crystal device described above operates as follows.
That is, a selection voltage is sequentially supplied to scanning lines to turn on all TFTs connected to predetermined scanning lines, such that all pixels corresponding to the scanning lines are selected. Then, an image signal is supplied to data lines in synchronization with selection of the pixels. Then, the image signal is supplied to all of the selected pixels through the TFTs that are in the ON state and an image voltage based on the image signal is written into pixel electrodes.
When the image voltage is written into the pixel electrodes, a driving voltage is applied to liquid crystal due to a potential difference between each pixel electrode and a common electrode. When the driving voltage is applied to liquid crystal, the alignment or ordering of molecules of the liquid crystal changes, and accordingly, light that is transmitted through the liquid crystal and is emitted from a backlight changes. When the changed light is transmitted through color filters, gray-scale display is performed. In addition, due to the storage capacitors, the driving voltage applied to liquid crystal is retained for a period of time that is three orders of magnitude longer than a period of time for which the image voltage is written.
The liquid crystal device described above is used for a portable apparatus, for example, and reduction of power consumption has been recently demanded in portable apparatuses. Therefore, JP-A-2002-196358 proposes a liquid crystal device capable of having a reduced power consumption by turning off TFTs and varying a voltage of a capacitive line after writing an image voltage into a pixel electrode.
Hereinafter, an operation of a liquid crystal device for varying a voltage of a capacitive line as disclosed in JP-A-2002-196358 will be described with reference to FIGS. 12 and 13. FIG. 12 is a timing chart illustrating the positive-polarity writing of a liquid crystal device in the related art. FIG. 13 is a timing chart illustrating the negative-polarity writing of a liquid crystal device in the related art. Here, the liquid crystal device in the related art has 320 rows of scanning lines and 240 columns of data lines, for example. In FIGS. 12 and 13, GATE(m) indicates a voltage of an m-th (where, ‘m’ is an integer satisfying 1≦m≦320) row scanning line out of 320 rows of scanning lines, and VST(m) indicates a voltage of an m-th row capacitive line out of 320 rows of capacitive lines. In addition, SOURCE(n) indicates a voltage of an n-th (where, ‘n’ is an integer satisfying 1≦n≦240) column data line out of 240 columns of data lines. In addition, PIX(m, n) indicates a voltage of a pixel electrode included in a pixel, which corresponds to an m-th row and an n-th column, provided at an intersection of an m-th row scanning line and an n-th column data line, and VCOM(m) indicates a voltage of a common electrode included in the pixel corresponding to the m-th row and the n-th column.
First, the positive-polarity writing of a liquid crystal device in the related art will now be described with reference to FIG. 12. At time t51, a scanning line driving circuit supplies a selection voltage to an m-th row scanning line. Accordingly, a voltage GATE(m) of the m-th row scanning line rises and then reaches a voltage VGH at time t52. As a result, all TFTs connected to the m-th row scanning line are turned on.
At time t53, a data line driving circuit supplies a positive-polarity image signal to an n-th column data line. Accordingly, a voltage SOURCE(n) of the n-th column data line rises gradually and then reaches a voltage VP8 at time t54. The voltage SOURCE(n) of the n-th column data line is written, as an image voltage based on the positive-polarity image signal, into the pixel electrode included in the pixel corresponding to the m-th row and the n-th column through a TFT 51 that is in the ON state and is connected to the m-th row scanning line. Accordingly, a voltage PIX(m, n) of the pixel electrode included in the pixel corresponding to the m-th row and the n-th column rises gradually and then reaches a voltage VP8, which is at the same potential level as the voltage SOURCE(n) of the n-th column data line, at time t54.
At time t55, the scanning line driving circuit stops supplying the selection voltage to the m-th row scanning line. Accordingly, the voltage GATE(m) of the m-th row scanning line drops and then reaches a voltage VGL at time t56. As a result, all TFTs 51 connected to the m-th row scanning line are turned off. At the same time, a capacitive line driving circuit supplies a voltage for increasing a voltage of a capacitive line to the m-th row capacitive line. Accordingly, a voltage VST(m) of the m-th row capacitive line rises gradually and then reaches a voltage VSTH at time t57. If the voltage VST(m) of the m-th row capacitive line rises, electric charges equivalent to the amount of voltage increase are distributed between a storage capacitor and a pixel capacitor in all pixels related to the m-th row capacitive line. Accordingly, a voltage PIX(m, n) of the pixel electrode included in the pixel corresponding to the m-th row and the n-th column rises gradually and then reaches a voltage VP9 at time t57.
Next, the negative-polarity writing of a liquid crystal device in the related art will be described with reference to FIG. 13. At time t61, a scanning line driving circuit supplies a selection voltage to an m-th row scanning line. Accordingly, a voltage GATE(m) of the m-th row scanning line rises and then reaches a voltage VGH at time t62. As a result, all TFTs connected to the m-th row scanning line are turned on.
At time t63, a data line driving circuit supplies a negative-polarity image signal to an n-th column data line. Accordingly, a voltage SOURCE(n) of the n-th column data line drops gradually and then reaches a voltage VP11 at time t64. The voltage SOURCE(n) of the n-th column data line is written, as an image voltage based on the negative-polarity image signal, into the pixel electrode included in the pixel corresponding to the m-th row and the n-th column through a TFT that is in the ON state and is connected to the m-th row scanning line. Accordingly, a voltage PIX(m, n) of the pixel electrode included in the pixel corresponding to the m-th row and the n-th column drops gradually and then reaches a voltage VP11, which is at the same potential level as the voltage SOURCE(n) of the n-th column data line, at time t64.
At time t65, the scanning line driving circuit stops supplying the selection voltage to the m-th row scanning line. Accordingly, the voltage GATE(m) of the m-th row scanning line drops and then reaches a voltage VGL at time t66. As a result, all TFTs connected to the m-th row scanning line are turned off. At the same time, a capacitive line driving circuit supplies a voltage for decreasing a voltage of a capacitive line to the m-th row capacitive line. Accordingly, a voltage VST(m) of the m-th row capacitive line drops gradually and then reaches a voltage VSTL at time t67. If the voltage VST(m) of the m-th row capacitive line drops, electric charges equivalent to the amount of voltage drop are distributed between a storage capacitor and a pixel capacitor in all pixels related to the m-th row capacitive line. Accordingly, a voltage PIX(m, n) of the pixel electrode included in the pixel corresponding to the m-th row and the n-th column drops gradually and then reaches a voltage VP10 at time t67.
As described above, in the liquid crystal device according to the related art, a voltage of a capacitive line rises after writing a positive-polarity image voltage into a pixel electrode at the time of the positive polarity writing. For this reason, the voltage of the pixel electrode rises by an amount obtained by adding a voltage, which is increased due to the positive-polarity image voltage, and a voltage increased due to electric charges by which a voltage of a capacitive line rises. Furthermore, in the liquid crystal device according to the related art, a voltage of a capacitive line drops after writing a negative-polarity image voltage into a pixel electrode at the time of the negative polarity writing. For this reason, the voltage of the pixel electrode drops by an amount obtained by adding a voltage, which is decreased due to the negative-polarity image voltage, and a voltage decreased due to electric charges by which a voltage of a capacitive line drops.
Therefore, by varying a voltage of the capacitive line to change a voltage of the pixel electrode with a voltage of the common electrode as a reference, it is possible to increase the amplitude of the driving voltage applied to liquid crystal. Thus, the amplitude of the driving voltage applied to liquid crystal is secured even if the amplitude of an image voltage is small. As a result, the power consumption may be reduced by making the amplitude of the image voltage small.
In the above liquid crystal device according to the related art, electric charges are made to move between a storage capacitor and a pixel capacitor by varying the voltage of a capacitive line, such that the voltage of a pixel electrode is varied. For this reason, in the case when variation in characteristics of the storage capacitor occurs, the electric charges moving between the storage capacitor and the pixel capacitor are affected. Accordingly, even if the same image voltage is written into the pixel electrode, variation occurs in the voltage of the pixel electrode, which causes the display quality to be degraded.
Further, in the case of an IPS (In-plane switching) liquid crystal device or FFS (fringe-field switching) liquid crystal device having a pixel electrode and a common electrode, which form a pixel capacitor, formed on one of a pair of substrates having liquid crystal interposed therebetween, a pixel capacitor and a storage capacitor are integrally formed. However, in the above liquid crystal device according to the related art, the voltage of the capacitive line is varied using a voltage different from the pixel electrode or the common electrode. Accordingly, it is necessary to form an electrode of the storage capacitor connected to the capacitive line separately from the pixel electrode or the common electrode. For this reason, the pixel capacitor and the storage capacitor need to be separately formed. Accordingly, in the case of the IPS liquid crystal device or the FFS liquid crystal device in which the pixel capacitor and the storage capacitor are integrally formed, to date, it has been difficult to form the above-described liquid crystal device according to the related art.